Magnetism

1. Why is a chunk of iron attracted to either pole of a magnet? What does this tell you about the reliability of attraction as a test of whether something is permanently magnetized or not? A classic puzzle involves two seemingly identical rods, one steel and magnetized (with poles at its ends), the other soft iron and not magnetized. How can you tell which rod is which, using nothing else and not bending or breaking either rod.

2. James Clerk Maxwell confirmed experimentally that the B-field of a long straight current-carrying wire drops off inversely with distance. His apparatus consists of a lightweight disk, free to rotate, on which rest for bar magnets. No matter how larege the current through the central wire, there was never any rotation of the disk. Given that the poles are at distances of Rs and Rn from the wire, use the idea of fictitious magnetic charges and the torques they would experience to explain the experiment.

3. An electric door bell is basically an automatically interrupted electromagnet. Describe how it works.

Multiple choice

1. A straight length of current-carrying wire is in uniform magnetic field. If the wire does not experience a force: (a) everything is as it should be (b) it must be parallel to B (c) we have an impossible situation (d) the wire must be perpendicular to B (e) none of these. Explain your choice.

2. The American Physicist Henry Rowland (1876) placed some charges (in a fixed location) on a nonconducting disk, which he then rotated at high speed abut its central axis near a delcate compass. (a) the current created a B-field that deflected the compass (b) there was no E-field, so nothing happened to the compass (c) the charges attracted the compass, which moved toward the disk (d) the charges repelled the compass via Coulomb's Law (e) none of these. Explain your choice.

Problem

1. A magnetic field of 6.0 u T is to be produced 10 cm from a single long straight wire in air. How much current must it carry?

2. A long straight wire carrrying a current of 4.00 A is suspended in air and the field around it is measured with a magnetometer. It is found that at some point P the field is 0.660 x 10^-5 T. How far from the wire is P?